1. Field of the Invention
The present invention relates to a schematic eye and an adjustment method and evaluation method for an optical coherence tomography apparatus.
2. Description of the Related Art
As an apparatus which noninvasively captures (obtains) a tomogram of a living tissue (for example, the eye's retina), an optical coherence tomography apparatus (to be sometimes referred to as an OCT hereinafter) is known.
The optical coherence tomography apparatus two-dimensionally scans a light beam over the retina via a deflector and measures the resultant reflected light and backscattered light with an interferometer. This procedure obtains a three-dimensional image including information in the depth (vertical) direction.
Conventionally, in order to improve the image quality of tomograms obtained by the OCT, efforts have been made to increase resolution (resolving power). The image quality (spatial resolving power) of a three-dimensional image is evaluated separately based on a horizontal resolving power representing the resolving power in a direction perpendicular to the optical-axis direction of a light beam irradiated from the OCT and a vertical resolving power representing the resolution in the optical-axis direction. For this reason, different techniques are used to evaluate the respective spatial resolving powers.
In an OCT and SLO (Scanning Laser Ophthalmoscope), the designed horizontal resolving power of an image of the fundus is determined by the diameter of a beam spot scanned over the retina. In a fundus camera, the designed horizontal resolving power is determined by the NA of an optical system.
Such evaluation of a designed horizontal resolving power generally uses a schematic eye. A schematic eye includes a single lens or a plurality of lenses and is provided with a resolution chart at a surface corresponding to the retina. In an apparatus as a target of evaluation of a horizontal resolving power, this technique captures an image of this pattern and calculates the density (intensity) contrast of the pattern corresponding to each spatial frequency. This procedure evaluates the evaluation target apparatus to determine whether it has achieved a desired horizontal resolving power.
For example, in the schematic eye disclosed in Japanese Patent Laid-Open No. 2002-165759, a lens is disposed into the main body of the almost cylindrical schematic eye, and a resolution chart is arranged at a fundus conjugate position at the time of optometry by a fundus camera. This schematic eye has a reflecting member for light diffusion only on the rear side of the chart. The operator adjusts the focus on an eye to be examined while watching the monitor. The horizontal resolving power is evaluated in an in-focus state.
A vertical resolving power δ of an OCT is theoretically obtained as the half width of the coherence function of an interferometer. More specifically, this resolving power is calculated byδ=2·ln(2)·λ02/(π·Δλ)  (1)where λ0 is the center wavelength of an irradiation light source and Δλ is the half width of a wavelength spectrum.
OCTs generally use low-coherence light sources. However, in order to reduce the value of δ, a light source with a large value of Δλ has been under development. Recently, it is reported that using a light source with Δλ of 100 nm or more has achieved the vertical resolving power δ of about 3 μm.
These values are merely theoretical values, and hence it is desired to evaluate a vertical resolving power as well as a horizontal resolving power by measuring actual values. As described above, the evaluation of a vertical resolving power does not use any resolution chart like that used for the evaluation of a horizontal resolving power. As a technique of evaluating a vertical resolving power using actual values, there is known a technique of measuring a plant cell, a multilayer film, or the like whose approximate order of size is known. The technique disclosed in T. Ralston et. al., “Real-time interferometric synthetic aperture microscopy”, Opt. Express (16) 2555-2569 (2008) forms a thin film by dispersing TiO2 particles with an average particle size of 1 μm in silicon and evaluates the image quality of a tomogram based on the size of an image of each particle.
The value of δ (the vertical resolving power of the OCT) obtained by equation (1) given above is based on the premise that the wavelength-spectrum distribution of a light source has a Gaussian shape. FIG. 7A shows graphs respectively indicating a light-source spectrum distribution with a Gaussian shape and a corresponding coherence-function shape. In this case, Δλ is 50 nm and derived δ is 6 μm.
In this case, for example, an SLD (Super Luminescent Diode) or the like is often used as a light source for the OCT. However, the spectrum distribution of an SLD often has a shape other than a Gaussian shape. FIG. 7B shows graphs respectively representing the spectrum distribution of an SLD and a corresponding coherence-function shape. In this case, the half width of the coherence function shown in the graph on the right side of FIG. 7B does not greatly differ from the value of δ obtained by equation (1), but the bottom portions of the curve greatly spread. That is, the sharpness decreases.
Although it depends on the specifications of an OCT, the sampling intervals in the vertical direction are practically limited to about several μm in many cases. For this reason, even evaluating a vertical resolving power by obtaining the width of a measured coherence function often fails to obtain the necessary accuracy.
A coherence function is the minimum unit in forming the vertical component of a tomogram, and corresponds to a point-image distribution function in an optical system, such as a camera, which forms two-dimensional images. Therefore, the vertical component of a tomogram distribution to be observed is obtained as the convolution between the actual scattering intensity distribution of an object to be examined and this coherence function.
The profiles of tomograms obtained by measuring a given object using OCTs respectively having the coherence functions shown in FIGS. 7A and 7B will be described below with reference to FIG. 8. In this case, the dotted line indicates an object having a rectangular periodic scattering-intensity distribution, the thin line indicates the profile of the tomogram obtained by measurement by the OCT having the coherence function in FIG. 7A, and the thick line indicates the profile of the tomograms obtained by measurement by the OCT having the coherence function in FIG. 7B.
As shown in FIG. 8, although the half widths of the coherence functions shown in FIGS. 7A and 7B are almost the same, the contrasts at positions in the images which correspond to high-frequency light greatly differ from each other, and the contrasts at low-depth positions (that is, shallow positions) decrease in difference. Therefore, δ (the half width of a coherence function) used as an index indicating a vertical resolving power is not necessarily appropriate as a value expressing a vertical resolving power when tomograms are visualized.